Method for the Solution Crystallization of Mixtures of Substances

ABSTRACT

A method for solution crystallization of mixtures of substances includes: providing a liquid mixture of substances including at least one component substance having an elevated melting point; drawing off a substream from the mixture of substances; feeding the substream to a heat exchanger so that the substream flows through the heat exchanger; cooling the substream in the heat exchanger to a temperature below a crystallization temperature of only the at least one component substance so that crystals from the at least one component substance form in the substream at an outlet of the heat exchanger; blending the substream including the crystals into the mixture of substances so that the crystals in the mixture of substances selectively bind other substances as a result of crystal growth; and providing for sedimentation and precipitation of the crystals in the mixture of substances.

The present invention relates to a method for the solution crystallization of liquid mixtures of substances in accordance with claim 1.

When working with liquid mixtures of substances, the fundamental problem generally arises that component substances that have a higher melting point than the other substances in the mixture of substances, crystallize out in response to an increased cooling of the mixture of substances. Thus, the mixture of substances becomes a suspension, in the case of increasing crystallization of one or more component substances, problems potentially arising with regard to manipulation of the mixture of substances.

In this regard, diesel fuels provide a typical example from day-to-day practice. In diesel fuels, specific component substances crystallize out at low winter temperatures, as a result, the mixture of substances macroscopically taking on a gelatinous substance having an increased viscosity and thereby substantially hindering a manipulation in injection components, such as in diesel engine nozzles. This limitation is particularly observed in what is commonly known as biodiesel, where the mixture of substances is formulated from oils having a different origin or preparation than component substances. Crystallization is typically prevented by selectively admixing appropriate additives into the mixture of substances, as is customarily the case in what is generally referred to as winter diesel, for example.

In the meantime, biodiesel production has gained a permanent place in the provision of regenerative liquid energy carriers. In its classic form, biodiesel is obtained by the esterification of rapeseed oil to acquire rapeseed oil methyl ester (RME). However, biodiesel can be produced more cost-effectively from recycled cooking oil (recycled cooking-oil methyl ester=AME). Due to the substantially higher concentration of saturated fatty acid methyl esters in the case of AME, this biodiesel can only be used in the summer.

To avoid the aforementioned crystallization in the biodiesel, current attempts are directed to improving the low-temperature resistance of recycled cooking oil-based biodiesel (AME). The low-temperature resistance refers to the temperature at which a substance changes from the liquid to the solid state of aggregation. If crystals are formed in the process, it corresponds to the melting temperature of the substance. In practical terms, this means that the fluidity of the fuel is limited by increasing gelation and by the formation of crystals in the product, making it unsuited for use in engines. Besides the properties of the fatty acid methyl esters, companion substances, such as waxes, residues of undesired reaction products, such as glycerides and glycerin, contribute to this behavior.

Existing methods for winterizing biodiesel entail blending with mineral diesel or adding additives to RME, for example. At present, the use of additives is the method of choice, at least in the case of rapeseed oil.

The mode of action of such products is based on the formation of a large number of crystallization nuclei having a size smaller than 50 μm which prevent an agglomeration in the biodiesel. A gelation or crystallization is effectively countered in this manner, so that the requirements of winterization (CFPP—measurement pursuant to DIN EN 116) are able to be formally met. However, the problem is not fundamentally resolved since a crystallization of the product is substantially retarded, but is not prevented.

Furthermore, available additives were specially developed for RME only. However, in the case of AME, they do not function in the manner discussed above. On the other hand, special additives for AME, which comply with the commercial restrictions specified for fuels, are not known.

Alternatively, U.S. Pat. No. 4,265,826 describes a fractionation-based winterization process and the physical filtering out of crystallized components, whereby the high-boiling components crystallize out as a function of their boiling points. In the process, the mixture of substances is continually cooled until crystals form in a size suitable for filtration. In this context, preferred cooling rates are between 0.5 and 5° C./h, at specific temperature levels, retention times of between 2-16 h being optionally provided. Depending on the particular low-temperature resistance requirement for the product, the total yield can fall below 30% relative to the starting substance, making the process uneconomical.

However, filtering processes have only limited effectiveness when the viscosity of the still liquid portion of the substance mixture increases and when the effectiveness of the separation subsides, particularly for the gelatinous crystals. Moreover, the aforementioned total yield is negatively affected by the liquid components of the mixture of substances that are bound in the crystals. However, a very long process duration of up to 100 hours and the large quantity of the mixture of substances to be selectively cooled, particularly in the case of diesel fuels, also make this approach seem hardly practicable from a standpoint of plant technology.

Moreover, in the field of classic winterization, the German Patent DE 34 44 475 C2 describes a device for the separation of crystals using a rotary-drum belt-type vacuum filter for the dry fractionation of fats and oils. The process is carried out batchwise and does not require any additional chemicals.

Against this background, an object of the present invention is to devise a method for the solution crystallization of mixtures of substances, particularly of fuels such as biodiesel, that does not require any additives and that is well suited for improving winterization, especially of biodiesel fuels, and for relatively large quantities as well.

This objective is achieved by a method having the features set forth in claim 1. Advantageous embodiments of this method are recited in the dependent claims.

The approach is based on a method for the solution crystallization of mixtures of substances, in particular of fuels such as biodiesel. In the process, a substream is drawn off from a liquid mixture of substances containing at least one component substance having an elevated melting point. This substream includes all of the component substances occurring in the mixture of substances and is preferably representative of the mixture of substances, i.e., it retains the same mutual proportion of the component substances as found in the mixture of substances. The substream is subsequently fed to a heat exchanger and introduced into the fluid guides thereof, the substream flowing through the heat exchanger. In the heat exchanger, the substream is cooled to a temperature below a crystallization temperature of only the component substances of the mixture of substances, while the other component substances in the substream remain unchanged in the liquid phase. In the process, the temperature and the flow rate are set to allow crystals from the component substances that have crystallized out to form in the substream directly at the outlet of the heat exchanger. In this context, the crystallization site may also be selectively controlled using fluid expansion means, such as a nozzle or a pressure reducer, for example. The substream having formed crystals of one or more component substances is subsequently reblended into the mixture of substances, the crystals in the mixture of substances selectively binding other substances to themselves due to crystal growth. In the process, to achieve an improved crystal growth, the mixture of substances is either not circulated (sedimentation) or intermixed, it is cooled to or retained at what is known as the cloud point, which is the temperature at which a turbidity and beginning crystallization may be ascertained. Once sedimentation is achieved and a predetermined size suitable for filtration is obtained, the crystals are separated out of the mixture of substances, for example by filtration or through the use of an ultracentrifuge.

The method is clarified in greater detail with reference to exemplary embodiments, test examples and the following figures, which show:

FIG. 1: the schematic design of an exemplary embodiment for implementing the method on a laboratory scale;

FIG. 2: the characteristic time curves of the AME concentration and of the temperature in the storage jar in accordance with the first test example;

FIG. 3: the characteristic time curves of the AME concentration and of the temperature in the storage jar in accordance with the second test example; as well as

FIG. 4: the characteristic curve of the AME bath temperature as a function of the cumulative concentration of saturated fatty acid methyl esters, as well as the empirically ascertained characteristic curve of the CFPP values (CFPP=cold filter plugging point; measurement pursuant to DIN EN 116) as a function of the cumulative concentration of saturated fatty acid methyl esters.

The exemplary embodiment in accordance with FIG. 1 includes a storage jar 1 for approximately five liters of biodiesel 2 having a temperature sensor 32, as well as a circuit including suction line 9, a pump 10 (gear-type pump), a heat exchanger 16 (in the exemplary embodiment, a cross-flow micro heat exchanger) and return flow line 20 for branching off the partial crystallization and for returning the substream from and into the storage jar. Storage jar 1, for example a five-liter beaker, includes a tempering device, in the exemplary embodiment, a wrap-around PVC tubing as cooling jacket 3, which, in the context of a closed coolant circuit, is connected via a coolant feed line 7 and a coolant return line 8 to a thermostat 5. A 1:1 volumetric blend of ethylene glycol and water is used as coolant 6. To provide efficient thermostatting, storage jar 1 and coolant feed line 7 are surrounded by insulation material. Biodiesel 2 in the storage jar is optionally kept in motion by a magnetic stirrer 4.

The substream is suctioned by pump 10 via suction line 9 (in the exemplary embodiment, a flexible metal tube or suction spout), preferably from the bottom third portion of storage jar 1, and returned again via return flow line 20 to the top third portion. A substantial portion of biodiesel 2 in storage jar 1 is circulated in this manner. Pump 10 delivers the substream via a heat exchanger conduit 12 (for example, flexible metal tube) having a microfilter 11 to heat exchanger 16. To maintain a constant heat-exchanger inlet temperature and to avoid premature crystallization in the substream, the heat exchanger conduit is equipped upstream of and in heat exchanger 16 with a tempering device for the substream. In the laboratory-scale exemplary embodiment, this tempering device includes a tube 15 (PVC tubing wrapping around the heat exchanger conduit), which is filled with tempering medium and is connected on both sides to a water thermostat 13.

Upon entry into heat exchanger 16 (cross-flow heat exchanger), the substream is split up among the fluid guides of the cross-flow heat exchanger and cooled by a cooling medium 22 (ethylene glycol/water) in the cross flow. With the aid of a cooling-medium pump 24, the cooling medium is conveyed through a cooling-medium thermostat 21 horizontally through fluid guides provided therefor in heat exchanger 16. The heat exchanger, cooling-medium thermostat, cooling-medium pump, as well as a coolant filter 25 are connected via a coolant feed line 23, a coolant line 26, a coolant outgoing line 27, respectively other lines (in each case polyethylene tubes) to a closed coolant circuit.

In the exemplary embodiment, a cross-flow micro heat exchanger having a multiplicity of fluid guides having diameters or maximum cross-sectional dimensions smaller than 1 mm is used as a heat exchanger. Due to its fineness, the heat exchanger features a high specific heat transfer, i.e., large specific heat-transfer surfaces and small heat-transfer paths. The fluid guides are divided into two fluid-guide portions that are layered in alternating sequence and cross one another preferably at a right angle. A micro heat exchanger is preferably used that has a layered structure including a metal foil stack, the fluid guides being incorporated as channel-shaped depressions in the individual foils that are stacked one over the other and diffusion welded together in a fluid-tight manner. In this context, each of these individual foils has a multiplicity of substantially identical, preferably mutually in parallel extending fluid guides of only one fluid guide portion.

After flowing through the heat exchanger, the biodiesel, which by this time is cooled, reaches heat exchanger outlet 17. In this region, the aforementioned crystals 19 form from saturated fatty acid methyl esters. This is assisted by the abrupt widening in cross section of the substream upon its exiting from the fluid guides, thereby spontaneously expanding the same. To be able to assess the quantity and appearance of crystals 19 in the substream, an inspection glass 18 is interposed between heat exchanger 16 and return flow line 20 leading to storage jar 1 for purposes of visual control (for example, by subjective visual inspection or using an optical method such as extinction or transmission measurements). In this context, a high nucleation rate is desired, i.e., to the extent possible, the smallest and most finely distributed crystals in the substream. These are introduced via the return flow line into the storage jar in the top third portion thereof, and are used as seed crystals for the biodiesel contained therein.

To precisely determine the crystallization conditions, the temperature is measured at the inlet and outlet of the biodiesel and cooling-medium passages of heat exchanger 16 with the aid of thermosensors (temperature sensors at heat-exchanger inlet 28, at heat-exchanger outlet 29, at the cooling-medium inflow 30 of the heat exchanger, as well as at the cooling-medium outflow 31 of the heat exchanger), and the temperature gradient in the heat exchanger is determined therefrom. Another temperature sensor 32 is located inside of storage jar 1. In addition, pressure sensors (pressure sensor at heat-exchanger inlet 33, as well as at heat-exchanger outlet 34) are placed at both passage inlets of heat exchanger 16, in order to be able to detect a potential incipient blockage in the heat exchanger.

This device makes it possible for biodiesel fuels to be winterized in the manner discussed at the outset, in two different ways, as described in the following:

TEST EXAMPLE 1

The temperature of the biodiesel is continually lowered in heat exchanger 16 by adjusting the temperature in coolant thermostat 21 until crystals 19 appear in inspection glass 18. The temperature of cooling jacket 3 of storage jar 1 is slowly lowered by adjusting the temperature in thermostat 5. The temperature of the refrigerant circuit in heat exchanger 16 is kept constant. The crystals formed are directed via the substream circuit into storage jar 1 and sediment there to the bottom, but are stirred up again by magnetic stirrer 4, thereby resulting in distribution of the crystals as crystallization nuclei throughout the entire storage jar. By collecting samples at regular intervals, the decrease in the saturated fatty acid methyl ester in the liquid biodiesel may be determined with the aid of gas chromatography. The temperature in the storage jar is then continually lowered until the desired composition of biodiesel 2 is reached.

TEST EXAMPLE 2

The temperature of the biodiesel is continually lowered in heat exchanger 16 by adjusting the temperature in coolant thermostat 21 until crystals 19 appear in inspection glass 18. The temperature of cooling jacket 3 of storage jar 1 is slowly lowered by adjusting the temperature in thermostat 5. Following a predetermined time for crystallization and crystal growth (approximately five hours), the two pumps 10 and, respectively, 24, and magnetic stirrer 4 are switched off. Crystals that have formed in storage jar 1 sink towards the bottom, while the biodiesel in the storage jar is sampled and analyzed at regular intervals. If the concentration of the saturated fatty acid methyl ester no longer changes or changes just slightly, the temperature of storage jar 1 is lowered further by thermostat 5. This process is continually repeated until the desired composition of the biodiesel is reached.

In the present exemplary embodiment, biodiesel derived from recycled cooking oil (AME) was used whose composition had been previously determined. The proportion of saturated fatty acid methyl ester at the beginning of the test was 22%.

FIG. 2 and FIG. 3 show the characteristic time curve of the cumulative concentration (concentration curve 38) of the saturated fatty acid methyl ester of AME and of the temperature in storage jar 1 (temperature curve 39) as functions of time for test example 1 (FIG. 2), respectively test example 2 (FIG. 3). In both diagrams, the biodiesel bath temperature 35 in ° C., as well as the concentration of AME 36 in % by weight are plotted over time 37 in minutes.

It should be noted, however, that the temperature had not always settled to its equilibrium value during sample collection. From these diagrams, a correlation between the temperature and a decrease in the saturated methyl ester may be discerned in storage jar 1. Over the entire measuring duration, one obtained a cooling rate of 0.02° C./h and a decrease in concentration of 0.03% by weight/h for test example 1; and a cooling rate of 0.04° C./h and a decrease in concentration of 0.06% by weight/h for test example 2. Thus, the same result was obtained in half of the time. From this result, the inference may be drawn that the time required for a winterization process may be substantially reduced by optimizing the cooling rate.

For both test examples, FIG. 4 illustrates the temperature curves (as a function of concentration 41, in each case as a point set and as a geometrically averaged straight-line segment; the straight-line segment from test example 1 resides below that from test example 2) in the storage jar of AME (biodiesel temperature 35), as well as the empirical characteristic curves of CFPP values 42 (CFPP value 40) as functions of the cumulative concentration of saturated fatty acid methyl esters (concentration 36). The CFPP is a measure of the winterization of the biodiesel. It increases with rising concentration of the saturated fatty acids. In both of the aforementioned test examples, care was taken during the measurements to ensure that bath temperatures were above the CFPP values for the same concentration of the saturated fatty acids.

One advantageous further refinement of the method provides for using a light-scattering photometer in order to precisely monitor the crystallization. In the case of a light-scattering photometer, light is coupled through an optical window into the biodiesel; the light scattered by the crystals is measured; and the crystal concentration is determined therefrom. Thus, the flow rate or the temperature may be regulated in such a way that a constant crystal concentration is always present.

To cool heat exchanger 16, instead of a liquid coolant, in this case an ethylene glycol/water blend, another refrigerant medium, such as R134a, may also be used, for example, that vaporizes inside of the heat exchanger, i.e., in the refrigerant-medium fluid guides. To avoid refrigerant medium losses, it is essential that the refrigerant-medium circuit be connected in a fluid-tight manner, the temperature during vaporization being adjustable as a function of the pressure prevailing in the refrigerant medium. Another advantageous refinement includes a use of counter-flow micro heat exchanger(s), instead of the cross-flow micro heat exchanger described in the exemplary embodiment. This makes it possible to achieve a more uniform temperature distribution at the outlet of the heat exchanger.

LIST OF REFERENCE NUMERALS

-   1 storage jar -   2 biodiesel -   3 cooling jacket -   4 magnetic stirrer -   5 thermostat -   6 coolant -   7 coolant feed line -   8 coolant return line -   9 suction line -   10 pump -   11 microfilter -   12 heat exchanger conduit -   13 water thermostat -   14 water -   15 tube -   16 heat exchanger -   17 heat-exchanger outlet -   18 inspection glass -   19 crystals -   20 return flow line -   21 cooling-medium thermostat -   22 cooling medium -   23 cooling-medium outflow -   24 cooling-medium pump -   25 cooling-medium filter -   26 cooling-medium line -   27 cooling-medium inflow -   28 temperature sensor at the heat-exchanger inlet -   29 temperature sensor at the heat-exchanger outlet -   30 temperature sensor at the cooling-medium inflow of the heat     exchanger -   31 temperature sensor at the cooling-medium outflow of the heat     exchanger -   32 temperature sensor in the storage jar -   33 pressure sensor at the heat-exchanger inlet -   34 pressure sensor at the heat-exchanger outlet -   35 biodiesel temperature -   36 concentration of AME -   37 time -   38 characteristic time curve of the concentration -   39 characteristic temperature curve over time -   40 CFPP value -   41 temperature curve as a function of the concentration -   42 empirical characteristic curves of the CFPP value 

1-12. (canceled)
 13. A method for solution crystallization of mixtures of substances, the method comprising: a) providing a liquid mixture of substances including at least one component substance having an elevated melting point; b) drawing off a substream from the mixture of substances; c) feeding the substream to a heat exchanger so that the substream flows through the heat exchanger; c) cooling the substream in the heat exchanger to a temperature below a crystallization temperature of only the at least one component substance so that crystals from the at least one component substance form in the substream at an outlet of the heat exchanger; d) blending the substream including the crystals into the mixture of substances so that the crystals in the mixture of substances selectively bind other substances as a result of crystal growth; and e) providing for sedimentation and precipitation of the crystals in the mixture of substances.
 14. The method as recited in claim 13, further comprising intermixing subsequent to the blending and prior to the providing for sedimentation and precipitation.
 15. The method as recited in claim 13, further comprising conveying the substream in the heat exchanger in fluid guides having diameters smaller than 1 mm.
 16. The method as recited in claim 14, further comprising conveying the substream in the heat exchanger in fluid guides having diameters smaller than 1 mm.
 17. The method as recited in claim 13, further comprising expanding the mixture of substances at the outlet of the heat exchanger.
 18. The method as recited in claim 14, further comprising expanding the mixture of substances at the outlet of the heat exchanger.
 19. The method as recited, in claim 13, further comprising tempering the heat exchanger by a fluid.
 20. The method as recited, in claim 14, further comprising tempering the heat exchanger by a fluid.
 21. The method as recited in claim 19, wherein the fluid includes a blend of ethylene glycol and water.
 22. The method as recited in claim 19, wherein the fluid has a boiling temperature point corresponding to the temperature below the crystallization temperature of only the at least one component substance and at which vaporization occurs in the heat exchanger.
 23. The method as recited in claim 13, wherein the mixture of substances is not intermixed and separately cooled during the crystal growth.
 24. The method as recited in claim 13, wherein the mixture of substances includes a blend of saturated and unsaturated methyl esters.
 25. The method as recited in claim 14, wherein the mixture of substances includes a blend of saturated and unsaturated methyl esters.
 26. The method as recited in claim 15, wherein the mixture of substances includes a blend of saturated and unsaturated methyl esters.
 27. The method as recited in claim 16, wherein the mixture of substances includes a blend of saturated and unsaturated methyl esters.
 28. The method as recited in claim 13, wherein the mixture of substances includes a fuel.
 29. The method as recited in claim 14, wherein the mixture of substances includes a fuel.
 30. The method as recited in claim 15, wherein the mixture of substances includes a fuel.
 31. The method as recited in claim 28, wherein the fuel includes a biodiesel including recycled cooking oils.
 32. The method as recited in claim 13, wherein the heat exchanger is a cross-flow micro heat exchanger or a counter-flow micro heat exchanger. 